‘Our relationship has not yet reached the point of no return.’
Photograph: Mode Images Limited/Alamy

As the storm clouds gather over Brexit, the EU withdrawal bill has been delayed a second time while the government tries to persuade backbench Tories to revoke their support for amendments that would allow MPs to block a “no deal” Brexit. The ever-sane Conservative MPs Dominic Grieve and Anna Soubry have put their names to an amendment that would provide that any final deal must be approved by a separate act of parliament. This is more than parliament parking its constitutional tanks on the government’s lawn: it means that if, as seems increasingly likely, the only option on offer is a disastrous, no-deal Brexit, MPs can require the government to think again.

These amendments prompt the question: haven’t we already bound ourselves to leave the EU, by triggering article 50? The answer is no, probably not. No one has ever tested exactly what article 50 means before, because no one has ever used it, so anything that a lawyer says about its reversibility is informed speculation. But most EU lawyers think that having given notification of intention to withdraw from the EU under article 50 doesn’t actually bind us to doing it.

Article 50 is a provision for withdrawal from the EU, not expulsion from it. We are the petitioners here. If parliament decides that having thought it through, we would be mad to leave and wants to call it off, then there is a short window in which the court of justice of the EU would probably rule that we can. That is, we can decide to stay unilaterally, without asking the European council, or the commission, or the EU27, for permission.

The frequently used divorce metaphor is helpful here. All we have done is tell the EU we are unhappy and plan to go. Our relationship has not yet reached the point of no return. True, we have thrown the wedding ring at them and said we’re off. We have instructed lawyers. We have lodged the petition. But we have not yet annulled the marriage. If the UK changes its mind before the end of the two-year notice period, then, (most lawyers think, not to mention Lord Kerr, who drafted article 50), we can simply say we have changed our minds, and we want to stay. EU law means never having to say you’re sorry.

‘The ever-sane Conservative MPs Dominic Grieve and Anna Soubry have put their names to an amendment that would provide that any final deal must be approved by a separate act of parliament.’ Photograph: Yui Mok/PA

At the moment, public opinion indeed seems resigned to leaving the EU. But that could change. Then MPs, too, may very rapidly decide that Theresa May was wrong to say that the best alternative to a poor deal is a disastrous no-deal Brexit. The best alternative is to change our mind and stay.

But whose mind matters: parliament’s, or the prime minister’s? Here the law is clear. It is parliament, and not the cabinet, which is in the driving seat. The Grieve amendment is not about asking for parliament to have a vote on the final outcome: it is about saving the government from themselves, by pointing out that, in the absence of parliamentary authority, the prime minister cannot take us out of the EU.

How are hard Brexiters responding to this? At the moment, by putting their fingers in their ears and saying “too late”. The transport secretary, Chris Grayling, told the Andrew Marr show last Sunday that parliament could have no mechanism to keep Britain in the EU beyond 30 March 2019. He claimed that “parliament has already voted for leaving the European Union”. He’s wrong – not only because no parliament can prevent a future one from changing its mind, but because parliament has not yet even voted to leave the EU: all that the prime minister has done so far is give notice of the current government’s present intention to leave.

After the referendum, Gina Miller and others challenged the government’s assertion that it could trigger Brexit on the basis of the referendum result alone, without further parliamentary authority. The supreme court pointed out that the EU Referendum Act had done no more than allow the government to hold an advisory referendum. It was parliament which made EU law part of UK law and so only parliament could take us out of the EU and stop EU law applying directly in the UK.

So under current law we are part of the EU, and ultimately, the decision to leave the EU must be taken by parliament. If parliament wishes, it can instruct the prime minister of the day – whoever he or she may be – to inform the EU that we will not, after all, be leaving. If the government tries to take us out of the EU without a deal approved by parliament, there will certainly be another outing for constitutional lawyers to refight the English civil war.

• Helen Mountfield is a QC at Matrix Chambers. She appeared for the people’s challenge litigants in the Gina Miller case.